1. Field of the Invention
The invention relates generally to compact solid state radar antenna structures, and more particularly, to microwave or millimeter wave radar antenna structures employing a folded optics path, a front transreflector, and a rear polarizing reflector.
2. Discussion of the Background Art
Solid state electronic microwave and millimeter wave radars generally contain three main components, a transceiver for generating and receiving electromagnetic radiation, an antenna for collimating and focusing the radiation, and a feed assembly for coupling the radiation from the transceiver to the antenna. In addition to collimating and focusing the emitted radiation, the antenna also receives and focuses the radiation reflected from detected objects, and provides it to the transceiver. To provide a compact structure, it is known to utilize a folded optics path, such that the longitudinal dimension of the antenna is less than the focal length of the lens alone. See, for example, U.S. Pat. No. 5,455,589 issued on Oct. 3, 1995 to Huguenin et al. and entitled "Compact Microwave and Millimeter Wave Radar". In the arrangement disclosed by Huguenin et al., a planoconvex lens is used to obtain radiation collimation.
In U.S. Pat. No. 4,599,623, issued to Havkin et al. on Jul. 8, 1986 and entitled "Polarizer Reflector and Reflecting Plate Scanning Antenna Including Same", there is disclosed an antenna which includes a front transreflector and a rear polarizer reflector, also known as a twist reflector. The transreflector is a polarizer, also known as an analyzer, that transmits radiation having a particular polarization, and that reflects radiation having a polarization orthogonal to the particular polarization. The front transreflector is a paraboloid, which transmits linear polarized, collimated electromagnetic (EM) radiation. In a receive mode, the paraboloid transreflector focuses linear polarized EM radiation. The twist reflector taught by Havkin et al. is flat and orthogonal to the axis of the paraboloid transreflector. As a consequence, the flat twist reflector arrangement reflects incident, linearly polarized EM radiation, and imparts a ninety degree twist such that the incident radiation is reflected with a polarization having an orthogonal ninety degree twist.
The antenna structure disclosed by Havkin et al. is a component of a microwave or millimeter wave EM radar in which the front transreflector is in front of an EM feed, at a focus of the antenna. The axis of the paraboloid transreflector and the axis of the orthogonal twist reflector must align to define properly the directions in which the antenna transmits and receives radiation. The need for this alignment gives rise to a problem in manufacturing. Specifically, alignment of the parabolic transreflector axis is made difficult because the orientation of the paraboloid changes as the transreflector undergoes thermal expansion and contraction with variations in temperature. Moreover, another disadvantage of the Havkin et al. arrangement resides in its required length. That is, the Havkin et al. structure requires that the EM feed be positioned at the actual focal length of the paraboloid, which limits any reduction of the actual length of the antenna.
Accordingly, there exists a need for a compact solid state radar antenna structure in which the possibility of improper or inefficient performance as a result of variations in temperature during manufacture and/or operation is reduced.